1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) device, and more particularly, to a backlight device for use in the LCD device.
2. Description of Related Art
FIG. 1 shows a conventional backlight device for use in a LCD device. As shown in FIG. 1, the conventional backlight device 1 comprises a light source 10, a U-shaped lamp holder having a lamp reflector 16, and a light waveguide plate 40. The light source 10 is located adjacent to an end 40a of the light waveguide plate 40, and most of the light source 10 is surrounded by the lamp holder 15. The end 40a constitutes a surface through which the incident light from the light source 10 is emitted.
The emitted light from the light source 10 is directed toward the surface 40a of the light waveguide plate 40 by the lamp reflector 16. The light waveguide plate 40 directs light either toward a prism sheet 20 or a reflector 50. The light reflected by the reflector 50 is emitted toward the observer through a prism sheet 20, a diffusing sheet 30 and a liquid crystal panel (not shown) in the form of an information.
The aforementioned backlight device is disclosed in the prior art, for example, the Japanese Patent Publication of Application No. 6-118246. The prior art prism sheet 20 has a plurality of convex portions 33 for collecting the light emitted from the light waveguide plate 40. The convex portions 33 of the prism sheet 20 can have a fan shape (FIG. 1), a regular triangle shape (FIG. 2a), an isosceles triangle shape (FIG. 2b), or a half-circle shape (FIG. 2c). Therefore, the prism sheet 20 having a plurality of the convex portions 33 collects the light emitted from the light waveguide plate 40 not to be scattered so that the regularly directional light is emitted toward the diffusing sheet 30.
However, even though the prism sheet 20 having a plurality of the convex portions 33 is arranged, light loss may occur. For example, as shown in FIG. 2a, part of the emitted light from the light waveguide plate 40 is total-reflected only to be directed downwardly, and then the light reflected on the reflector 50 again is directed upwardly, having a large refraction angle with respect to the vertical direction.
Further, if the prism sheet is used together with a cholesteric liquid crystal; (CLC) polarizer, the color change property and light efficiency may decrease. In general, a directional vector of a CLC layer has a helical structure. When a right-handed circularly polarized lightis incident to the CLC layer having a-left-handed helical structure, the CLC layer reflects only a light having a wavelength that satisfies a Bragg reflection condition. Using such a principle, the CLC polarizer can use even light lost in an absorbing type polarizer, thereby remarkably improving light efficiency. In the same way, when a CLC color filter is used, the light efficiency is much improved.
However, when an incident angle of the circularly polarized light is greater than 20 degrees, the transmitted light is produced in the form of not circularly polarized light, but eliptically polarized light. Besides, a wavelength of the transmitted light is different from that of the incident light, thereby lessening the color change property and the light efficiency.
An object of the present invention is to provide a backlight device for a liquid crystal display device such that the light collecting efficiency and light illumination are much improved.
In order to achieve the above object, the present invention provides a backlight device for a liquid crystal display device, which includes a light source; a light waveguide plate having an end portion and a front portion; a lamp holder having a lamp reflector, the lamp holder surrounding most part of the light source, the lamp reflector reflecting light to the end portion of the light waveguide plate; and a first prism sheet located on the front portion of the light waveguide plate, the prism sheet having a plurality of convex portions, the convex portions having an inclined line portion and a curved line portion, the inclined line portion having a predetermined angle with respect to a direction vertical to the front portion of the light waveguide plate.
The angle of the inclined line portion is less than 20xc2x0. The light waveguide plate has a plurality of convex or concave hole dots. The light waveguide plate has a plurality of V-shaped convex or concave bars.
The backlight device further includes a reflector, arranged under the light waveguide plate and a diffusing sheet, interposed between the light waveguide plate and the first prism sheet; a second prism sheet having a plurality of convex portions, the second prism sheet arranged on the first prism sheet; a cholesteric liquid crystal polarizer arranged on the second prism sheet; and a quarter wave plate arranged on the cholesteric liquid crystal polarizer.